1. Field of the Invention
The invention relates generally to electrical switching apparatus and, more particularly, to stored energy assemblies for electrical switching apparatus, such as circuit breakers. The invention also relates to time delay mechanisms for circuit breaker stored energy assemblies.
2. Background Information
Electrical switching apparatus, such as circuit breakers, provide protection for electrical systems from electrical fault conditions such as, for example, current overloads, short circuits, abnormal voltage and other fault conditions. Typically, circuit breakers include an operating mechanism which opens electrical contact assemblies to interrupt the flow of current through the conductors of an electrical system in response to such fault conditions as detected, for example, by a trip unit.
Some medium voltage circuit breakers, for example, employ a spring-operated stored energy assembly. Specifically, the operating mechanism of such circuit breakers typically includes an opening assembly having at least one spring which facilitates the opening (e.g., separation) of the electrical contact assemblies, a closing assembly including a number of springs that close the electrical contact assemblies, and a charging mechanism for charging the spring(s). The contact assemblies are closed by releasing the stored energy of the closing assembly spring(s). The closing assembly spring(s) is/are charged either manually, using a manual charging mechanism such as, for example, a charging handle, or automatically using, for example, a motor-driven charging mechanism or other suitable electromechanical charging mechanism. Each of the manual and automatic charging mechanisms of known stored energy assemblies requires its own individual “chain” or assembly of components, in order to link the corresponding power source (e.g., human power; motor power) to the spring(s) that must be charged. There are numerous components in each of these assemblies, some of which are relatively complex to make and/or are difficult to install or assemble. Additionally, the components of the manual and automatic charging mechanisms, as well as the other components of the stored energy assembly in general, are typically “built in” with respect to the circuit breaker. In other words, they are individually coupled to various locations on the circuit breaker housing and not readily interchangeable for use in other applications (e.g., with other circuit breakers). This makes it difficult to repair, replace and/or maintain the charging mechanisms because to do so requires the entire circuit breaker to be at least partially disassembled. Moreover, the charging handle for the manual charging mechanism is a relatively large (e.g., long, in order to provide leverage) separate component, which is typically not permanently attached and, therefore, must be stored separate from the circuit breaker, and can be lost.
Another disadvantage with respect to the stored energy assemblies of some circuit breakers deals with the timing (e.g., delay) of the opening of the electrical contact assemblies in response to the fault condition. Specifically, an electronic trip circuit monitors the load currents and, if any of these currents exceeds certain current-time characteristics, then an opening trigger mechanism such as, for example, an opening solenoid is actuated to trip open the electrical contact assemblies. It is generally assumed that the response time of the trigger mechanism should be as short as possible, in order to separate the electrical contact assemblies as quickly as possible and avoid, or minimize, damage to the circuit breaker and/or load side electrical components. However, it has been determined that if the contact assemblies are separated when both the direct and alternating currents that are associated with the fault condition are at or near their peak values, then the contacts may be damaged. Delaying separation of the contacts until the direct current is off peak can avoid such damage. The problem is that the particular time delay, which is optimal, is different for different applications and different circuit breakers.
There is, therefore, room for improvement in electrical switching apparatus, such as circuit breakers, and in stored energy assemblies therefor. There is also room for improvement in time delay mechanisms for circuit breaker stored energy assemblies.